1. Field of the Invention
The present invention relates to a semiconductor laser device, and a method for producing the same. In particular, the present invention relates to a semiconductor laser device having a window structure for providing an improved production yield, and a method for producing the same.
2. Description of the Related Art
In recent years, semiconductor laser devices for use as light sources for information processing apparatuses (e.g., optical disk apparatuses) have been expected to provide higher and higher output power in order to attain higher recording speeds. One method for satisfying such needs involves the use of a window structure at an end face of a laser cavity, which improves the catastrophic optical damage (hereinafter “COD”) level at the end face.
COD is an instantaneous degradation phenomenon which occurs as the optical output of a semiconductor laser is increased to or above a certain limit value. A COD phenomenon occurs in the case where the vicinity of an end face of a semiconductor laser becomes an absorption region with respect to the light which occurs within the laser. Specifically, a COD phenomenon may occur when dangling bonds are formed due to oxygen adsorption or surface oxidation on the semiconductor surface of the end face of the laser cavity, generating a peculiar deep level on the semiconductor surface, thereby substantially narrowing the forbidden band width in the vicinity of the end face. Since any non-radiative recombination due to a surface level present on the semiconductor surface introduces an increase in temperature, the occurrence of a COD phenomenon results in further reduction of the forbidden band width in the vicinity of the end faces of the laser cavity and further increases light absorption. Thus, there is a positive feedback with respect to the COD phenomenon. As a result, the end faces may be destroyed due to melting or the like, which leads to a decrease in the optical output as well as irreversible degradation of the device.
Some semiconductor laser devices employ a window structure so as to enhance the band gap energy in a portion of an active layer near a laser cavity end face, thereby preventing COD destruction at the cavity end face. For example, a window structure can be realized by diffusing an impurity in the vicinity of a laser cavity end face so as to disorder the super-lattice structure in the active layer.
Hereinafter, a method for producing a conventional window structure semiconductor laser device will be described. FIGS. 6A to 6D are diagrams illustrating respective steps of a manufacturing process for a conventional window structure semiconductor laser device.
First, as shown in FIG. 6A, the following semiconductor multilayer structure is formed on an n-GaAs substrate 601 by using an MOVPE (metal organic vapor phase epitaxy) method, where the respective layers are sequentially grown into crystals in this order: an n-AlGaInP cladding layer 602, an AlGaInP/GaInP super-lattice active layer 603, a p-AlGaInP first cladding layer 604, a p-GaInP etching stop layer 605, a p-AlGaInP second cladding layer 606, a p-GaInP band graded layer 607, and a p-GaAs capping layer 608.
Next, as shown in FIG. 6B, by using a plasma CVD (chemical vapor deposition) technique, an SiN film 609 is formed on the aforementioned semiconductor multilayer structure. Furthermore, the SiN film 609 is patterned by dry etching so as to form two parallel openings whose planar forms appear as two stripes having a width of several dozen μm. A wet etching step removes the portions of the GaAs capping layer 608 where these openings are formed. Thereafter, a ZnO film 610 and an SiO2 film 611 are formed by sputtering so as to cover the semiconductor multilayer structure (including the openings). Furthermore, an annealing is performed so as to diffuse Zn from the ZnO film 610 through the portions of the p-GaInP layer 607 which are exposed in the openings of the SiN film 609, and the openings in the GaAs capping layer 608. Through such solid-phase diffusion of Zn, impurity diffusion regions 612 having stripe-like planar forms are formed, and the portions of the AlGaInP/GaInP super-lattice active layer 603 which lie within the impurity diffusion regions 612 are converted into a mixed crystal. The regions of the active layer 603 which have been converted into the mixed crystal define window structures. The window structures have a higher band gap energy than that of the regions which have not formed a mixed crystal.
Referring to FIG. 6C, the SiO2 film 611, the ZnO film 610, the SiN film 609, and the GaAs film 608 are removed. Thereafter, by using a known technique, a stripe pattern of SiO2 film 613 is formed on the exposed p-GaInP band graded layer 607 so as to extend along a plane which is perpendicular to the longitudinal direction of the impurity diffusion regions 612. By using the stripe pattern of SiO2 film 613 as a mask, the p-GaInP band graded layer 607 is etched into a ridge shape by using an acetic acid-type etchant. Then, switching to a sulfuric acid-type etchant, the p-AlGaInP second cladding layer 606 is etched away until reaching the p-GaInP etching stop layer 605. As a result, a ridge structure composed of the p-GaInP band graded layer 607 and the p-AlGaInP second cladding layer 606 is formed as shown in FIG. 6C. Since the sulfuric acid-type etchant has a greater etching rate for the p-AlGaInP second cladding layer 606 than for the p-GaInP etching stop layer 605, the etching process can be successfully stopped at the etching stop layer 605.
Thereafter, an n-type current blocking layer 614 is grown so as to bury the side of the ridge structure by a selective growth technique using an MOVPE method. After removing the SiO2 film 613 serving as a stripe mask, a p-GaAs contact layer 615 is grown over the n-type current blocking layer 614 and the p-GaInP band graded layer 607. By using a known technique, p-side and n-side ohmic electrodes are formed (not shown).
The resultant semiconductor multilayer structure is cleaved in the impurity diffusion regions 612 along a plane perpendicular to the longitudinal direction of the ridge structure, thereby forming laser cavity end faces. As a result, a semiconductor laser device having window structures as shown in FIG. 6D is accomplished.
Conventional semiconductor laser devices with window structures are formed by the above-described manufacturing process. However, in accordance with the above-described manufacturing process, not only the active layer 603 but also the p-GaInP etching stop layer 605 are converted into a mixed crystal together with the surrounding AlGaInP layers during the step of Zn diffusion. That is, in accordance with above-described conventional manufacturing process, Zn is directly diffused from the Zn source, i.e., the ZnO film 610, into the AlGaInP layers, which have a relatively large diffusion coefficient. Therefore, it is difficult to control the impurity dose amount. As a result, as shown in FIG. 1, for example, a high concentration of impurity is diffused in the AlGaInP crystal, allowing for a rapid development of the mixed crystal. In particular, the thin etching stop layer 605 may eventually be destroyed by the etching. In that case, since the etching selection ratio of the sulfuric acid-type etchant is extremely decreased, the etching cannot be stopped by the etching stop layer 605, allowing the p-AlGaInP first cladding layer 604 and the active layer 603 to be etched. Thus, the ridge shape may not be controlled properly.
Moreover, in accordance with the above-described conventional manufacturing process, the current blocking layer 614 may be formed so as to be nearer the active layer 603 due to overetching. As a result, the angle of expanse of light exiting the active layer 603 cannot be effectively controlled. In a loss-guide type semiconductor laser device, in particular, the propagation loss is increased so that the laser characteristics of the device are greatly deteriorated, resulting in, e.g., a decrease in the output power, or an increase in the operation current.
Furthermore, in accordance with the above-described conventional manufacturing process, the impurity concentration in the active layer 603 becomes very high as shown in FIG. 1. As a result, the propagation loss due to carrier scattering is increased so that the laser characteristics of the device are greatly deteriorated, resulting in, e.g., a decrease in the output power, or an increase in the operation current.
As a method for avoiding overetching, for example, Japanese Laid-Open Publication No. 9-139550 discloses a method which involves first exposing the AlGaInP layer 606 by removing the p-GaInP layer 607 while leaving portions of the p-GaInP layer 607 only in regions where Zn has been diffused, and then performing an etching with a sulfuric acid-type etchant. According to this method, at the point in time where the etching has reached the etching stop layer 605 in regions other than the Zn diffusion regions, the AlGaInP layer 606 is left in the Zn diffusion regions due to the low etching rate for the p-GaInP band graded layer 607, and the etching is terminated at this point.
However, it is evidently difficult to accurately control the etching amount for the AlGaInP layer 606. The remainder of the AlGaInP layer 606 after etching varies per every etching.
Therefore, the height of the ridge which is formed as a result of the etching may vary, thus making it difficult to control the ridge shape.